Semi-crystalline consumable materials for electrophotography-based additive manufacturing system

ABSTRACT

A part material for printing three-dimensional parts with an electrophotography-based additive manufacturing system, the part material including a composition having a semi-crystalline thermoplastic material and a charge control agent. The part material is provided in a powder form having a controlled particle size, and is configured for use in the electrophotography-based additive manufacturing system having a layer transfusion assembly for printing the three-dimensional parts in a layer-by-layer manner.

BACKGROUND

The present disclosure relates to additive manufacturing systems forprinting three-dimensional (3D) parts and support structures. Inparticular, the present disclosure relates to consumable materials forprinting 3D parts and support structures using an imaging process, suchas electrophotography.

Additive manufacturing systems are used to build 3D parts from digitalrepresentations of the 3D parts (e.g., AMF and STL format files) usingone or more additive manufacturing techniques. Examples of commerciallyavailable additive manufacturing techniques include extrusion-basedtechniques, ink jetting, selective laser sintering, powder/binderjetting, electron-beam melting, and stereolithographic processes. Foreach of these techniques, the digital representation of the 3D part isinitially sliced into multiple horizontal layers. For each sliced layer,a tool path is then generated, which provides instructions for theparticular additive manufacturing system to form the given layer.

For example, in an extrusion-based additive manufacturing system, a 3Dpart or model may be printed from a digital representation of the 3Dpart in a layer-by-layer manner by extruding a flowable part material.The part material is extruded through an extrusion tip carried by aprint head of the system, and is deposited as a sequence of roads on asubstrate in an x-y plane. The extruded part material fuses topreviously deposited part material, and solidifies upon a drop intemperature. The position of the print head relative to the substrate isthen incremented along a z-axis (perpendicular to the x-y plane), andthe process is then repeated to form a 3D part resembling the digitalrepresentation.

In fabricating 3D parts by depositing layers of a part material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of objects under construction, whichare not supported by the part material itself. A support structure maybe built utilizing the same deposition techniques by which the partmaterial is deposited. The host computer generates additional geometryacting as a support structure for the overhanging or free-space segmentsof the 3D part being formed, and in some cases, for the sidewalls of the3D part being formed. The support material adheres to the part materialduring fabrication, and is removable from the completed 3D part when theprinting process is complete.

In two-dimensional (2D) printing, electrophotography (i.e., xerography)is a popular technology for creating 2D images on planar substrates,such as printing paper. Electrophotography systems include a conductivesupport drum coated with a photoconductive material layer, where latentelectrostatic images are formed by charging and then image-wise exposingthe photoconductive layer by an optical source. The latent electrostaticimages are then moved to a developing station where toner is applied tocharged areas of the photoconductive insulator to form visible images.The formed toner images are then transferred to substrates (e.g.,printing paper) and affixed to the substrates with heat or pressure.

SUMMARY

An aspect of the present disclosure is directed to a part material forprinting 3D parts with an electrophotography-based additivemanufacturing system. The part material has a composition that includesa semi-crystalline thermoplastic material and a charge control agent.The part material is provided in a powder form having a controlledparticle size (e.g., a D50 particle size ranging from about 5micrometers to about 30 micrometers), and is configured for use in theelectrophotography-based additive manufacturing system having a layertransfusion assembly for printing the 3D parts in a layer-by-layermanner. In some embodiments, the part material may be provided in aninterchangeable cartridge or other similar device, along with carrierparticles, for use with the electrophotography-based additivemanufacturing system.

Another aspect of the present disclosure is directed to a method forprinting a 3D part with an electrophotography-based additivemanufacturing system having an electrophotography engine, a transfermedium, and a layer transfusion assembly. The method includes providinga part material to the electrophotography-based additive manufacturingsystem, where the part material compositionally includes a chargecontrol agent and a semi-crystalline thermoplastic material, and has apowder form. In some embodiments, the part material may be provided inan interchangeable cartridge or other similar device, along with carrierparticles, for use with the electrophotography-based additivemanufacturing system.

The method also includes triboelectrically charging the part material toa desired triboelectric charge (e.g., a Q/M ratio having a negativecharge or a positive charge, and a magnitude ranging from about 5micro-Coulombs/gram to about 50 micro-Coulombs/gram), and developinglayers of the 3D part from the charged part material with theelectrophotography engine. The method further includes electrostaticallyattracting the developed layers from the electrophotography engine tothe transfer medium, moving the attracted layers to the layertransfusion assembly with the transfer medium, and transfusing the movedlayers to previously-printed layers of the 3D part with the layertransfusion assembly.

DEFINITIONS

Unless otherwise specified, the following terms as used herein have themeanings provided below:

The term “copolymer” refers to a polymer having two or more monomerspecies, and includes terpolymers (i.e., copolymers having three monomerspecies).

The terms “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from theinventive scope of the present disclosure.

Reference to “a” chemical compound refers one or more molecules of thechemical compound, rather than being limited to a single molecule of thechemical compound. Furthermore, the one or more molecules may or may notbe identical, so long as they fall under the category of the chemicalcompound. Thus, for example, “a” polyamide is interpreted to include oneor more polymer molecules of the polyamide, where the polymer moleculesmay or may not be identical (e.g., different molecular weights and/orisomers).

The terms “at least one” and “one or more of” an element are usedinterchangeably, and have the same meaning that includes a singleelement and a plurality of the elements, and may also be represented bythe suffix “(s)” at the end of the element. For example, “at least onepolyamide”, “one or more polyamides”, and “polyamide(s)” may be usedinterchangeably and have the same meaning.

Directional orientations such as “above”, “below”, “top”, “bottom”, andthe like are made with reference to a direction along a printing axis ofa 3D part. In the embodiments in which the printing axis is a verticalz-axis, the layer-printing direction is the upward direction along thevertical z-axis. In these embodiments, the terms “above”, “below”,“top”, “bottom”, and the like are based on the vertical z-axis. However,in embodiments in which the layers of 3D parts are printed along adifferent axis, the terms “above”, “below”, “top”, “bottom”, and thelike are relative to the given axis.

The term “providing”, such as for “providing a material” and the like,when recited in the claims, is not intended to require any particulardelivery or receipt of the provided item. Rather, the term “providing”is merely used to recite items that will be referred to in subsequentelements of the claim(s), for purposes of clarity and ease ofreadability.

Unless otherwise specified, temperatures referred to herein are based onatmospheric pressure (i.e. one atmosphere).

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an example electrophotography-based additivemanufacturing system for printing 3D parts from part materials of thepresent disclosure, along with associated support structures fromsupport materials.

FIG. 2 is a schematic front view of a pair of electrophotography enginesof the system for developing layers of the part and support materials.

FIG. 3 is a schematic front view of an alternative electrophotographyengine, which includes an intermediary drum or belt.

FIG. 4 is a schematic front view of a layer transfusion assembly of thesystem for performing layer transfusion steps with the developed layers.

DETAILED DESCRIPTION

The present disclosure is directed to semi-crystalline consumablematerials, which are engineered for use in an electrophotography-basedadditive manufacturing system to print 3D parts with high resolutionsand fast printing rates. During a printing operation, anelectrophotography (EP) engine may develop or otherwise image each layerof the part (and any associated support material) using theelectrophotography process. The developed layers are then transferred toa layer transfusion assembly where they are transfused (e.g., using heatand/or pressure) to print one or more 3D parts and support structures ina layer-by-layer manner.

In comparison to 2D printing, in which developed toner particles can beelectrostatically transferred to printing paper by placing an electricalpotential through the printing paper, the multiple printed layers in a3D environment effectively prevents the electrostatic transfer of partand support materials after a given number of layers are printed (e.g.,about 15 layers). Instead, each layer may be heated to an elevatedtransfer temperature, and then pressed against a previously-printedlayer (or to a build platform) to transfuse the layers together in atransfusion step. This allows numerous layers of 3D parts and supportstructures to be built vertically, beyond what is otherwise achievablevia electrostatic transfers.

As discussed below, the consumable material is a powder-based partmaterial derived from one or more semi-crystalline thermoplasticmaterials, a charge control agent, preferably a heat absorbent (e.g., aninfrared absorber) if required, and optionally one or more additionalmaterials, such as a flow control agent, which may also function as anexternal surface-treatment triboelectric charge control agent and/or atriboelectric modification additive. The part material is engineered foruse with electrophotography-based additive manufacturing systems toprint 3D parts having high part resolutions and good physical properties(e.g., good part strength, density, chemical resistance, usabletemperature ranges, and the like). This allows the resulting 3D parts tofunction as end-use parts, if desired.

The part material of the present disclosure is preferably printed alongwith a powder-based support material that is engineered to complementthe part materials. For example, each layer of the support material ispreferably transfused along with an associated layer of the partmaterial. As such, the support material preferably has a melt rheologythat is similar to, or more preferably substantially the same as, themelt rheology of its associated part material.

FIGS. 1-4 illustrate system 10, which is an exampleelectrophotography-based additive manufacturing system for printing 3Dparts and associated support structures with the part of the presentdisclosure, and associated support materials. As shown in FIG. 1, system10 includes a pair of EP engines 12 p and 12 s, belt transfer assembly14, biasing mechanisms 16 and 18, and layer transfusion assembly 20.Examples of suitable components and functional operations for system 10include those disclosed in Hanson et al., U.S. Publication Nos.2013/0077996 and 2013/0077997, and in Comb et al., U.S. Publication Nos.2013/0186549 and 2013/0186558.

EP engines 12 p and 12 s are imaging engines for respectively imaging orotherwise developing layers of the part and support materials, where thepart and support materials are each preferably engineered for use withthe particular architecture of EP engine 12 p or 12 s. As discussedbelow, the imaged layers may then be transferred to belt transferassembly 14 (or other transfer medium) with biasing mechanisms 16 and18, and carried to layer transfusion assembly 20 to print the 3D partsand associated support structures in a layer-by-layer manner.

In the shown embodiment, belt transfer assembly 14 includes transferbelt 22, belt drive mechanisms 24, belt drag mechanisms 26, loop limitsensors 28, idler rollers 30, and belt cleaner 32, which are configuredto maintain tension on belt 22 while belt 22 rotates in the rotationaldirection of arrows 34. In particular, belt drive mechanisms 24 engageand drive belt 22, and belt drag mechanisms 26 may function as brakes toprovide a service loop design for protecting belt 22 against tensionstress, based on monitored readings via loop limit sensors 28.

System 10 also includes controller 36, which is one or more controlcircuits, microprocessor-based engine control systems, and/ordigitally-controlled raster imaging processor systems, and which isconfigured to operate the components of system 10 in a synchronizedmanner based on printing instructions received from host computer 38.Host computer 38 is one or more computer-based systems configured tocommunicate with controller 36 to provide the print instructions (andother operating information). For example, host computer 38 may transferinformation to controller 36 that relates to the sliced layers of the 3Dparts and support structures, thereby allowing system 10 to print the 3Dparts and support structures in a layer-by-layer manner.

The components of system 10 may be retained by one or more framestructures, such as frame 40. Additionally, the components of system 10are preferably retained within an enclosable housing (not shown) thatprevents ambient light from being transmitted to the components ofsystem 10 during operation.

FIG. 2 illustrates EP engines 12 p and 12 s, where EP engine 12 s (i.e.,the upstream EP engine relative to the rotational direction of belt 22)develops layers of the support material, and EP engine 12 p (i.e., thedownstream EP engine relative to the rotational direction of belt 22)develops layers of the part material. In alternative embodiments, thearrangement of EP engines 12 p and 12 s may be reversed such that EPengine 12 p is upstream from EP engine 12 s relative to the rotationaldirection of belt 22. In further alternative embodiments, system 10 mayinclude three or more EP engines for printing layers of additionalmaterials.

In the shown embodiment, EP engines 12 p and 12 s may include the samecomponents, such as photoconductor drum 42 having conductive drum body44 and photoconductive surface 46. Conductive drum body 44 is anelectrically-conductive drum (e.g., fabricated from copper, aluminum,tin, or the like) that is electrically grounded and configured to rotatearound shaft 48. Shaft 48 is correspondingly connected to drive motor50, which is configured to rotate shaft 48 (and photoconductor drum 42)in the direction of arrow 52 at a constant rate.

Photoconductive surface 46 is a thin film extending around thecircumferential surface of conductive drum body 44, and is preferablyderived from one or more photoconductive materials, such as amorphoussilicon, selenium, zinc oxide, organic materials, and the like. Asdiscussed below, surface 46 is configured to receive latent-chargedimages of the sliced layers of a 3D part or support structure (ornegative images), and to attract charged particles of the part orsupport material to the charged or discharged image areas, therebycreating the layers of the 3D part or support structure.

As further shown, EP engines 12 p and 12 s also includes charge inducer54, imager 56, development station 58, cleaning station 60, anddischarge device 62, each of which may be in signal communication withcontroller 36. Charge inducer 54, imager 56, development station 58,cleaning station 60, and discharge device 62 accordingly define animage-forming assembly for surface 46 while drive motor 50 and shaft 48rotate photoconductor drum 42 in the direction of arrow 52.

In the shown example, the image-forming assembly for surface 46 of EPengine 12 s is used to form layers 64 s of the support material of thepresent disclosure (referred to as support material 66 s), where asupply of support material 66 s may be retained by development station58 (of EP engine 12 s) along with carrier particles. Similarly, theimage-forming assembly for surface 46 of EP engine 12 p is used to formlayers 64 p of the part material of the present disclosure (referred toas part material 66 p), where a supply of part material 66 p may beretained by development station 58 (of EP engine 12 p) along withcarrier particles.

Charge inducer 54 is configured to generate a uniform electrostaticcharge on surface 46 as surface 46 rotates in the direction of arrow 52past charge inducer 54. Suitable devices for charge inducer 54 includecorotrons, scorotrons, charging rollers, and other electrostaticcharging devices.

Imager 56 is a digitally-controlled, pixel-wise light exposure apparatusconfigured to selectively emit electromagnetic radiation toward theuniform electrostatic charge on surface 46 as surface 46 rotates in thedirection of arrow 52 past imager 56. The selective exposure of theelectromagnetic radiation to surface 46 is directed by controller 36,and causes discrete pixel-wise locations of the electrostatic charge tobe removed (i.e., discharged to ground), thereby forming latent imagecharge patterns on surface 46.

Suitable devices for imager 56 include scanning laser (e.g., gas orsolid state lasers) light sources, light emitting diode (LED) arrayexposure devices, and other exposure device conventionally used in 2Delectrophotography systems. In alternative embodiments, suitable devicesfor charge inducer 54 and imager 56 include ion-deposition systemsconfigured to selectively directly deposit charged ions or electrons tosurface 46 to form the latent image charge pattern. As such, as usedherein, the term “electrophotography” includes ionography.

Each development station 58 is an electrostatic and magnetic developmentstation or cartridge that retains the supply of part material 66 p orsupport material 66 s, preferably in powder form, along with carrierparticles. Development stations 58 may function in a similar manner tosingle or dual component development systems and toner cartridges usedin 2D electrophotography systems. For example, each development station58 may include an enclosure for retaining the part material 66 p orsupport material 66 s and carrier particles. When agitated, the carrierparticles generate triboelectric charges to attract the powders of thepart material 66 p or support material 66 s, which charges the attractedpowders to a desired sign and magnitude, as discussed below.

Each development station 58 may also include one or more devices fortransferring the charged part or support material 66 p or 66 s tosurface 46, such as conveyors, fur brushes, paddle wheels, rollers,and/or magnetic brushes. For instance, as surface 46 (containing thelatent charged image) rotates from imager 56 to development station 58in the direction of arrow 52, the charged part material 66 p or supportmaterial 66 s is attracted to the appropriately charged regions of thelatent image on surface 46, utilizing either charged area development ordischarged area development (depending on the electrophotography modebeing utilized). This creates successive layers 64 p or 64 s asphotoconductor drum 12 continues to rotate in the direction of arrow 52,where the successive layers 64 p or 64 s correspond to the successivesliced layers of the digital representation of the 3D part or supportstructure.

The successive layers 64 p or 64 s are then rotated with surface 46 inthe direction of arrow 52 to a transfer region in which layers 64 p or64 s are successively transferred from photoconductor drum 42 to belt22, as discussed below. While illustrated as a direct engagement betweenphotoconductor drum 42 and belt 22, in some preferred embodiments, EPengines 12 p and 12 s may also include intermediary transfer drumsand/or belts, as discussed further below.

After a given layer 64 p or 64 s is transferred from photoconductor drum42 to belt 22 (or an intermediary transfer drum or belt), drive motor 50and shaft 48 continue to rotate photoconductor drum 42 in the directionof arrow 52 such that the region of surface 46 that previously held thelayer 64 p or 64 s passes cleaning station 60. Cleaning station 60 is astation configured to remove any residual, non-transferred portions ofpart or support material 66 p or 66 s. Suitable devices for cleaningstation 60 include blade cleaners, brush cleaners, electrostaticcleaners, vacuum-based cleaners, and combinations thereof.

After passing cleaning station 60, surface 46 continues to rotate in thedirection of arrow 52 such that the cleaned regions of surface 46 passdischarge device 62 to remove any residual electrostatic charge onsurface 46, prior to starting the next cycle. Suitable devices fordischarge device 62 include optical systems, high-voltagealternating-current corotrons and/or scorotrons, one or more rotatingdielectric rollers having conductive cores with applied high-voltagealternating-current, and combinations thereof.

Transfer belt 22 is a transfer medium for transferring the developedsuccessive layers 64 p and 64 s from photoconductor drum 42 (or anintermediary transfer drum or belt) to layer transfusion assembly 16.Examples of suitable transfer belts for belt 22 include those disclosedin Comb et al., U.S. Publication Nos. 2013/0186549 and 2013/0186558.Belt 22 includes front surface 22 a and rear surface 22 b, where frontsurface 22 a faces surface 46 of photoconductor drum 42 and rear surface22 b is in contact with biasing mechanisms 16 and 18.

Biasing mechanisms 16 and 18 are configured to induce electricalpotentials through belt 22 to electrostatically attract layers 64 p and64 s from EP engines 12 p and 12 s to belt 22. Because layers 64 p and64 s are each only a single layer increment in thickness at this pointin the process, electrostatic attraction is suitable for transferringlayers 64 p and 64 s from EP engines 12 p and 12 s to belt 22.

Controller 36 preferably rotates photoconductor drums 36 of EP engines12 p and 12 s at the same rotational rates that are synchronized withthe line speed of belt 22 and/or with any intermediary transfer drums orbelts. This allows system 10 to develop and transfer layers 64 p and 66s in coordination with each other from separate developer images. Inparticular, as shown, each part layer 64 p may be transferred to belt 22with proper registration with each support layer 64 s to produce acombined part and support material layer 64. As can be appreciated, somelayers transferred to layer transfusion assembly 20 may only includesupport material 66 s or may only include part material 66 p, dependingon the particular support structure and 3D part geometries and layerslicing.

In an alternative embodiment, part layers 64 p and support layers 64 smay optionally be developed and transferred along belt 22 separately,such as with alternating layers 64 p and 64 s. These successive,alternating layers 64 p and 64 s may then be transferred to layertransfusion assembly 20, where they may be transfused separately toprint the 3D part and support structure.

In a further alternative embodiment, one or both of EP engines 12 p and12 s may also include one or more intermediary transfer drums and/orbelts between photoconductor drum 42 and belt 22. For example, as shownin FIG. 3, EP engine 12 p may also include intermediary drum 42 a thatrotates an opposing rotational direction from arrow 52, as illustratedby arrow 52 a, under the rotational power of motor 50 a. Intermediarydrum 42 a engages with photoconductor drum 42 to receive the developedlayers 64 p from photoconductor drum 42, and then carries the receiveddeveloped layers 64 p and transfers them to belt 22.

EP engine 12 s may include the same arrangement of intermediary drum 42a for carrying the developed layers 64 s from photoconductor drum 42 tobelt 22. The use of such intermediary transfer drums or belts for EPengines 12 p and 12 s can be beneficial for thermally isolatingphotoconductor drum 42 from belt 22, if desired.

FIG. 4 illustrates an example embodiment for layer transfusion assembly20. As shown, layer transfusion assembly 20 includes build platform 68,nip roller 70, heaters 72 and 74, post-fuse heater 76, and air jets 78(or other cooling units). Build platform 68 is a platform assembly orplaten of system 10 that is configured to receive the heated combinedlayers 64 (or separate layers 64 p and 64 s) for printing a 3D part andsupport structure, referred to as 3D part 80 and support structure 82,in a layer-by-layer manner. In some embodiments, build platform 68 mayinclude removable film substrates (not shown) for receiving the printedlayers 64, where the removable film substrates may be restrained againstbuild platform using any suitable technique (e.g., vacuum drawing).

Build platform 68 is supported by z-axis gantry 84, which is a guidemechanism configured to move build platform 68 along the z-axis and thex-axis to produce a reciprocating rectangular pattern, where the primarymotion is back-and-forth along the x-axis (illustrated by broken lines86. Gantry 84 may be operated by motor 88 based on commands fromcontroller 36, where motor 88 may be an electrical motor, a hydraulicsystem, a pneumatic system, or the like.

In the shown embodiment, build platform 68 is heatable with heatingelement 90 (e.g., an electric heater). Heating element 90 is configuredto heat and maintain build platform 68 at an elevated temperature thatis greater than room temperature (25° C.), such as at a desired averagepart temperature of 3D part 80 and/or support structure 82, as discussedin Comb et al., U.S. Publication Nos. 2013/0186549 and 2013/0186558.This allows build platform 68 to assist in maintaining 3D part 80 and/orsupport structure 82 at this average part temperature.

Nip roller 70 is an example heatable element or heatable layertransfusion element, which is configured to rotate around a fixed axiswith the movement of belt 22. In particular, nip roller 70 may rollagainst rear surface 22 b in the direction of arrow 92 while belt 22rotates in the direction of arrow 34. In the shown embodiment, niproller 70 is heatable with heating element 94 (e.g., an electricheater). Heating element 94 is configured to heat and maintain niproller 70 at an elevated temperature that is greater than roomtemperature (25° C.), such as at a desired transfer temperature forlayers 64.

Heater 72 is one or more heating devices (e.g., an infrared heaterand/or a heated air jet) configured to heat layers 64 to a temperaturenear an intended transfer temperature of the thermoplastic-based powder,such as at least a fusion temperature of the thermoplastic-based powder,prior to reaching nip roller 70. Each layer 64 desirably passes by (orthrough) heater 72 for a sufficient residence time to heat the layer 64to the intended transfer temperature. Heater 74 may function in the samemanner as heater 72, and heats the top surfaces of 3D part 80 andsupport structure 82 to an elevated temperature, such as at the sametransfer temperature as the heated layers 64 (or other suitable elevatedtemperature).

As mentioned above, the support material of the present disclosure usedto print support structure 82 preferably has a melt rheology that issimilar to or substantially the same as the melt rheology part materialof the present disclosure used to print 3D part 80. This allows part andsupport materials of layers 64 p and 64 s to be heated together withheater 74 to substantially the same transfer temperature, and alsoallows the part and support materials at the top surfaces of 3D part 80and support structure 82 to be heated together with heater 74 tosubstantially the same temperature. Thus, the part layers 64 p and thesupport layers 64 s may be transfused together to the top surfaces of 3Dpart 80 and support structure 82 in a single transfusion step ascombined layer 64.

Post-fuse heater 76 is located downstream from nip roller 70 andupstream from air jets 78, and is configured to heat the transfusedlayers to an elevated temperature in the post-fuse or heat-setting step.Again, the close melt rheologies of the part and support materials allowpost-fuse heater 76 to post-heat the top surfaces of 3D part 80 andsupport structure 82 together in a single post-fuse step.

Prior to printing 3D part 80 and support structure 82, build platform 68and nip roller 70 may be heated to their desired temperatures. Forexample, build platform 68 may be heated to the average part temperatureof 3D part 80 and support structure 82 (due to the close melt rheologiesof the part and support materials). In comparison, nip roller 70 may beheated to a desired transfer temperature for layers 64 (also due to theclose melt rheologies of the part and support materials).

During the printing operation, belt 22 carries a layer 64 past heater72, which may heat the layer 64 and the associated region of belt 22 tothe transfer temperature. Suitable transfer temperatures for the partand support materials of the present disclosure include temperaturesthat exceed the glass transition temperature of the part and supportmaterials, where the layer material is softened but not melted, forexample, a temperature of ranging from about 180° C. to about 250° C.for a polyamide-based part material.

As further shown in FIG. 4, during operation, gantry 84 may move buildplatform 68 (with 3D part 80 and support structure 82) in areciprocating rectangular pattern 86. In particular, gantry 84 may movebuild platform 68 along the x-axis below, along, or through heater 74.Heater 74 heats the top surfaces of 3D part 80 and support structure 82to an elevated temperature, such as the transfer temperatures of thepart and support materials. As discussed in Comb et al., U.S.Publication Nos. 2013/0186549 and 2013/0186558, heaters 72 and 74 mayheat layers 64 and the top surfaces of 3D part 80 and support structure82 to about the same temperatures to provide a consistent transfusioninterface temperature. Alternatively, heaters 72 and 74 may heat layers64 and the top surfaces of 3D part 80 and support structure 82 todifferent temperatures to attain a desired transfusion interfacetemperature.

The continued rotation of belt 22 and the movement of build platform 68align the heated layer 64 with the heated top surfaces of 3D part 80 andsupport structure 82 with proper registration along the x-axis. Gantry84 may continue to move build platform 68 along the x-axis, at a ratethat is synchronized with the rotational rate of belt 22 in thedirection of arrow 34 (i.e., the same directions and speed). This causesrear surface 22 b of belt 22 to rotate around nip roller 70 to nip belt22 and the heated layer 64 against the top surfaces of 3D part 80 andsupport structure 82. This presses the heated layer 64 between theheated top surfaces of 3D part 80 and support structure 82 at thelocation of nip roller 70, which at least partially transfuses heatedlayer 64 to the top layers of 3D part 80 and support structure 82.

As the transfused layer 64 passes the nip of nip roller 70, belt 22wraps around nip roller 70 to separate and disengage from build platform68. This assists in releasing the transfused layer 64 from belt 22,allowing the transfused layer 64 to remain adhered to 3D part 80 andsupport structure 82. Maintaining the transfusion interface temperatureat a transfer temperature that is higher than its glass transitiontemperature, but lower than its fusion temperature, allows the heatedlayer 64 to be hot enough to adhere to 3D part 80 and support structure82, while also being cool enough to readily release from belt 22.Additionally, as discussed above, the close melt rheologies of the partand support materials allow them to be transfused in the same step.

After release, gantry 84 continues to move build platform 68 along thex-axis to post-fuse heater 76. At post-fuse heater 76, the top-mostlayers of 3D part 80 and support structure 82 (including the transfusedlayer 64) may then be heated to at least the fusion temperature of thethermoplastic-based powder in a post-fuse or heat-setting step. Thismelts the material of the transfused layer 64 to a highly fusable statesuch that polymer molecules of the transfused layer 64 quicklyinterdiffuse to achieve a high level of interfacial entanglement with 3Dpart 80 and support structure 82.

Additionally, as gantry 84 continues to move build platform 68 along thex-axis past post-fuse heater 76 to air jets 78, air jets 78 blow coolingair towards the top layers of 3D part 80 and support structure 82. Thisactively cools the transfused layer 64 down to the average parttemperature, as discussed in Comb et al., U.S. Publication Nos.2013/0186549 and 2013/0186558.

To assist in keeping 3D part 80 and support structure 82 at the averagepart temperature, in some preferred embodiments, heater 74 and/orpost-heater 76 may operate to heat only the top-most layers of 3D part80 and support structure 82. For example, in embodiments in whichheaters 72, 74, and 76 are configured to emit infrared radiation, 3Dpart 80 and support structure 82 may include heat absorbers and/or othercolorants configured to restrict penetration of the infrared wavelengthsto within the top-most layers. Alternatively, heaters 72, 74, and 76 maybe configured to blow heated air across the top surfaces of 3D part 80and support structure 82. In either case, limiting the thermalpenetration into 3D part 80 and support structure 82 allows the top-mostlayers to be sufficiently transfused, while also reducing the amount ofcooling required to keep 3D part 80 and support structure 82 at theaverage part temperature.

Gantry 84 may then actuate build platform 68 downward, and move buildplatform 68 back along the x-axis to a starting position along thex-axis, following the reciprocating rectangular pattern 86. Buildplatform 68 desirably reaches the starting position for properregistration with the next layer 64. In some embodiments, gantry 84 mayalso actuate build platform 68 and 3D part 80/support structure 82upward for proper registration with the next layer 64. The same processmay then be repeated for each remaining layer 64 of 3D part 80 andsupport structure 82.

After the printing operation is completed, the resulting 3D part 80 andsupport structure 82 may be removed from system 10 and undergo one ormore post-printing operations. For example, support structure 82 may besacrificially removed from 3D part 80 using an aqueous-based solution,such as an aqueous alkali solution. Under this technique, supportstructure 82 may at least partially dissolve in the solution, separatingit from 3D part 80 in a hands-free manner.

In comparison, the part material is chemically resistant to aqueousalkali solutions. This allows the use of an aqueous alkali solution tobe employed for removing the sacrificial support structure 82 withoutdegrading the shape or quality of 3D part 80. Examples of suitablesystems and techniques for removing support structure 82 in this mannerinclude those disclosed in Swanson et al., U.S. Pat. No. 8,459,280;Hopkins et al., U.S. Pat. No. 8,246,888; and Dunn et al., U.S.Publication No. 2011/0186081; each of which are incorporated byreference to the extent that they do not conflict with the presentdisclosure.

Furthermore, after support structure 82 is removed, 3D part 80 mayundergo one or more additional post-printing processes, such as surfacetreatment processes. Examples of suitable surface treatment processesinclude those disclosed in Priedeman et al., U.S. Pat. No. 8,123,999;and in Zinniel, U.S. Publication No. 2008/0169585.

As briefly discussed above, the part material of the present disclosurecompositionally includes one or more semi-crystalline thermoplasticmaterials, such as one or more polyamides, polyethylenes,polypropylenes, semi-crystalline polyolefin block copolymers,popolyoxymethylenes, polyetherarylketones (e.g., polyetheretherketones),and mixtures thereof. In some embodiments, the semi-crystallinethermoplastic materials have heat deflection temperatures below about100° C. In other embodiments, the semi-crystalline thermoplasticmaterials have heat deflection temperatures ranging from about 100° C.to about 150° C. (e.g., engineering-grade thermoplastic materials). Infurther embodiments, the semi-crystalline thermoplastic materials haveheat deflection temperatures great than about 150° C. (e.g.,high-performance thermoplastic materials). Examples of suitablesemi-crystalline thermoplastic materials are discussed further below inconjunction with associated techniques for producing powders of the partmaterials.

As used herein, semi-crystalline thermoplastic materials havemeasureable melting points (5 calories/gram or more) using differentialscanning calorimetry (DSC) pursuant to ASTM D3418-08. Furthermore, thesemi-crystalline thermoplastic materials are preferably polymericmaterials (e.g., polymers) capable of exhibiting an average percentcrystallinity in a solid state of at least about 10% by weight, andinclude polymeric materials having crystallinities up to 100% (i.e.,fully-crystalline polymeric materials). In comparison, amorphousthermoplastic materials have substantially no measurable melting points(less than 5 calories/gram) using DSC pursuant to ASTM D3418-08.

As mentioned above, the part material is engineered for use in anEP-based additive manufacturing system (e.g., system 10) to print 3Dparts (e.g., 3D part 80). As such, the part material may also includeone or more materials to assist in developing layers with EP engine 12p, to assist in transferring the developed layers from EP engine 12 p tolayer transfusion assembly 20, and to assist in transfusing thedeveloped layers with layer transfusion assembly 20.

For example, in the electrophotographic process with system 10, the partmaterial is preferably charged triboelectrically through the mechanismof frictional contact charging with carrier particles at developmentstation 58. This charging of the part material may be referred to by itstriboelectric charge-to-mass (Q/M) ratio, which may be a positive ornegative charge and has a desired magnitude. The Q/M ratio is inverselyproportional to the powder density of the part material, which can bereferred to by its mass per unit area (M/A) value. For a given applieddevelopment field, as the value of Q/M ratio of the part material isincreased from a given value, the M/A value of the part materialdecreases, and vice versa. Thus, the powder density for each developedlayer of the part material is a function of the Q/M ratio of the partmaterial.

It has been found that, in order to provide successful and reliabledevelopment of the part material onto development drum 44 and transferto layer transfusion assembly 20 (e.g., via belt 22), and to print 3Dpart 80 with a good material density, the part material preferably has asuitable Q/M ratio for the particular architecture of EP engine 12 p andbelt 22. Examples of preferred Q/M ratios for the part material rangefrom about −5 micro-Coulombs/gram (μC/g) to about −50 μC/g, morepreferably from about −10 μC/g to about −40 μC/g, and even morepreferably from about −15 μC/g to about −35 μC/g, and even morepreferably from about −25 μC/g to about −30 μC/g.

In this embodiment, the Q/M ratio is based on a negative triboelectriccharge. However, in an alternative embodiment, system 10 may operatesuch that the Q/M ratio of the part material has a positivetriboelectric charge with the above-discussed magnitudes. In eitherembodiment, these magnitudes of Q/M ratio prevent the electrostaticforces constraining the part material to the carrier surfaces from beingtoo excessive, and that any level of “wrong sign” powder is minimized.This reduces inefficiencies in the development of the part material atEP engine 12 p, and facilitates the development and transfer of eachlayer 64 p with the desired M/A value.

Furthermore, if a consistent material density of 3D part 80 is desired,the desired Q/M ratio (and corresponding M/A value) is preferablymaintained at a stable level during an entire printing operation withsystem 10. However, over extended printing operations with system 10,development station 58 may need to be replenished with additionalamounts of the part material. This can present an issue because, whenintroducing additional amounts of the part material to developmentstation 58 for replenishment purposes, the part material is initially inan uncharged state until mixing with the carrier particles. As such, thepart material also preferably charges to the desired Q/M ratio at arapid rate to maintain a continuous printing operation with system 10.

Accordingly, controlling and maintaining the Q/M ratio during initiationof the printing operation, and throughout the duration of the printingoperation, will control the resultant rate and consistency of the M/Avalue of the part material. In order to reproducibly and stably achievethe desired Q/M ratio, and hence the desired M/A value, over extendedprinting operations, the part material preferably includes one or morecharge control agents, which may be added to the copolymer during themanufacturing process of the part material.

In embodiments in which the Q/M ratio of the part material has anegative charge, suitable charge control agents for use in the partmaterial include acid metal complexes (e.g., oxy carboxylic acidcomplexes of chromium, zinc, and aluminum), azo metal complexes (e.g.,chromium azo complexes and iron azo complexes), mixtures thereof, andthe like.

Alternatively, in embodiments in which the Q/M ratio of the partmaterial has a positive charge, suitable charge control agents for usein the part material include azine-based compounds, and quaternaryammonium salts, mixtures thereof, and the like. These agents areeffective at positively charging the copolymer when frictionally contactcharged against appropriate carrier particles.

The charge control agents preferably constitute from about 0.1% byweight to about 5% by weight of the part material, more preferably fromabout 0.5% by weight to about 2% by weight, and even more preferablyfrom about 0.75% by weight to about 1.5% by weight, based on the entireweight of the part material. As discussed above, these charge controlagents preferably increase the charging rate of the copolymer againstthe carrier, and stabilize the Q/M ratio over extended continuousperiods of printing operations with system 10.

In many situations, system 10 prints layers 64 p with a substantiallyconsistent material density over the duration of the printingoperations. Having a part material with a controlled and consistent theQ/M ratio allows this to be achieved. However, in some situations, itmay be desirable to adjust the material density between the variouslayers 64 p in the same printing operation. For example, system 10 maybe operated to run in a grayscale manner with reduced material density,if desired, for one or more portions of 3D part 80.

In addition to incorporating the charge control agents, for efficientoperation EP engine 12 p, and to ensure fast and efficient triboelectriccharging during replenishment of the part material, the mixture of thepart material preferably exhibits good powder flow properties. This ispreferred because the part material is fed into a development sump(e.g., a hopper) of development station 58 by auger, gravity, or othersimilar mechanism, where the part material undergoes mixing andfrictional contact charging with the carrier particles.

As can be appreciated, blockage or flow restrictions of the partmaterial during the replenishment feeding can inhibit the supply of thepart material to the carrier particles. Similarly, portions of the partmaterial should not become stuck in hidden cavities in developmentstation 58. Each of these situations can alter the ratio of the partmaterial to the carrier particles, which, as discussed above, ispreferably maintained at a constant level to provide the desired Q/Mratio for the charged part material.

For example, the part material may constitute from about 1% by weight toabout 30% by weight, based on a combined weight of the part material andthe carrier particles, more preferably from about 5% to about 20%, andeven more preferably from about 5% to about 10%. The carrier particlesaccordingly constitute the remainder of the combined weight.

The powder flow properties of the part material can be improved orotherwise modified with the use of one or more flow control agents, suchas inorganic oxides. Examples of suitable inorganic oxides includehydrophobic fumed inorganic oxides, such as fumed silica, fumed titania,fumed alumina, mixtures thereof, and the like, where the fumed oxidesmay be rendered hydrophobic by silane and/or siloxane-treatmentprocesses. Examples of commercially available inorganic oxides for usein the part material include those under the tradename “AEROSIL” fromEvonik Industries AG, Essen, Germany.

The flow control agents (e.g., inorganic oxides) preferably constitutefrom about 0.1% by weight to about 10% by weight of the part material,more preferably from about 0.2% by weight to about 5% by weight, andeven more preferably from about 0.3% by weight to about 1.5% by weight,based on the entire weight of the part material.

As discussed above, the one or more charge control agents are suitablefor charging the part material to a desired Q/M ratio for developinglayers of the part material at EP engine 12 p, and for transferring thedeveloped layers (e.g., layers 64) to layer transfusion assembly 20(e.g., via belt 22). However, the multiple printed layers in a 3Denvironment effectively prevents the electrostatic transfer of partmaterial after a given number of layers are printed. Instead, layertransfusion assembly 20 utilizes heat and pressure to transfuse thedeveloped layers together in the transfusion steps.

In particular, heaters 72 and/or 74 may heat layers 64 and the topsurfaces of 3D part 80 and support structure 82 to a temperature near anintended transfer temperature of the part material, such as at least afusion temperature of the part material, prior to reaching nip roller70. Similarly, post-fuse heater 76 is located downstream from nip roller70 and upstream from air jets 78, and is configured to heat thetransfused layers to an elevated temperature in the post-fuse orheat-setting step.

Accordingly, the part material may also include one or more heatabsorbers configured to increase the rate at which the part material isheated when exposed to heater 72, heater 74, and/or post-heater 76. Forexample, in embodiments in which heaters 72, 74, and 76 are infraredheaters, the heat absorber(s) used in the part material may be one ormore infrared (including near-infrared) wavelength absorbing materials.As discussed below, these heat absorbers may be incorporated into theparticles of the copolymer during the manufacturing of the partmaterial. Absorption of infrared light causes radiationless decay ofenergy to occur within the particles, which generates heat in the partmaterial.

The heat absorber is preferably soluble or dispersible in the solvatedcopolymers used for the preparation of the part material with a limitedcoalescence process, as discussed below. Additionally, the heat absorberalso preferably does not interfere with the formation of the copolymerparticles, or stabilization of these particles during the manufacturingprocess. Furthermore, the heat absorber preferably does not interferewith the control of the particle size and particle size distribution ofthe copolymer particles, or the yield of the copolymer particles duringthe manufacturing process.

Suitable infrared absorbing materials for use in the part material mayvary depending on the desired color of the part material. Examples ofsuitable infrared absorbing materials include carbon black (which mayalso function as a black pigment for the part material), as well asvarious classes of infrared absorbing pigments and dyes, such as thosethat exhibit absorption in the wavelengths ranging from about 650nanometers (nm) to about 900 nm, those that exhibit absorption in thewavelengths ranging from about 700 nm to about 1,050 nm, and those thatexhibit absorption in the wavelengths ranging from about 800 nm to about1,200 nm. Examples of these pigments and dyes classes includeanthraquinone dyes, polycyanine dyes metal dithiolene dyes and pigments,tris aminium dyes, tetrakis aminium dyes, mixtures thereof, and thelike.

The infrared absorbing materials also preferably do not significantlyreinforce or otherwise alter the melt rheological properties of thesemi-crystalline thermoplastic material, such as the zero shearviscosity versus temperature profile of the semi-crystallinethermoplastic material. For example, this can be achieved using anon-reinforcing type of carbon black, or a “low structure” type ofcarbon black, at low concentrations relative to the semi-crystallinethermoplastic material.

Accordingly, in embodiments that incorporate heat absorbers, the heatabsorbers (e.g., infrared absorbers) preferably constitute from about0.5% by weight to about 10% by weight of the part material, morepreferably from about 1% by weight to about 5% by weight, and in somemore preferred embodiments, from about 2% by weight to about 3% byweight, based on the entire weight of the part material.

The part material may also include one or more additional additives thatpreferably do not interfere with the formation of the thermoplasticparticles, or stabilization of these particles during the manufacturingprocess, and that preferably do not interfere with the control of theparticle size and particle size distribution of the thermoplasticparticles, or the yield of the thermoplastic particles during themanufacturing process.

Examples of suitable additional additives include colorants (e.g.,pigments and dyes in addition to, or alternatively to, the heatabsorbers), polymer stabilizers (e.g., antioxidants, light stabilizers,ultraviolet absorbers, and antiozonants), biodegradable additives, andcombinations thereof. In embodiments that incorporate additionaladditives, the additional additives may collectively constitute fromabout 0.1% by weight to about 10% by weight of the part material, morepreferably from about 0.2% by weight to about 5% by weight, and evenmore preferably from about 0.5% by weight to about 2% by weight, basedon the entire weight of the part material.

For use in electrophotography-based additive manufacturing systems(e.g., system 10), the part material preferably has a controlled averageparticle size and a narrow particle size distribution, as describedbelow in the Particle Sizes and Particle Size Distributions standard.For example, preferred D50 particles sizes include those up to about 100micrometers if desired, more preferably from about 10 micrometers toabout 30 micrometers, more preferably from about 10 micrometers to about20 micrometers, and even more preferably from about 10 micrometers toabout 15 micrometers.

Additionally, the particle size distributions, as specified by theparameters D90/D50 particle size distributions and D50/D10 particle sizedistributions, each preferably range from about 1.00 to 1.40, morepreferably from about 1.10 and to about 1.35, and even more preferablyfrom about 1.15 to about 1.25. Moreover, the particle size distributionis preferably set such that the geometric standard deviation σ_(g)preferably meets the criteria pursuant to the following Equation 1:

${\sigma \; g} \sim \frac{D\; 90}{D\; 50} \sim \frac{D\; 50}{D\; 10}$

In other words, the D90/D50 particle size distributions and D50/D10particle size distributions are preferably the same value or close tothe same value, such as within about 10% of each other, and morepreferably within about 5% of each other.

The part material is preferably manufactured by polymerizing orotherwise providing the semi-crystalline thermoplastic material(s), andthen formulating the part material from the semi-crystallinethermoplastic material(s) (and other components) with theabove-discussed particle sizes and particle size distributions. Theparticular formulation technique, however, is dependent on thesemi-crystalline thermoplastic material utilized in the part material.

For instance, when the semi-crystalline thermoplastic material is apolyamide material, the part material may be produced with a millingprocess, a spray-drying process, or a limited coalescence process.Following the milling or spray-drying process, the resulting powder mayundergo one or more additional classification processes to attain thedesired particle sizes and particle size distribution, if required.Examples of suitable milling systems for the milling process includethose commercially available under the tradename “IMD GRAN-U-LIZERS”from Modern Process Equipment Corporation, Chicago, Ill.

In some embodiments, the milling process may be performed as acryomilling process. For example, a feedstock of the polyamide(s), thecharge control agent, any heat absorber, and/or any additional additivemay be suspended as a cryogen slurry (e.g., in liquid nitrogen or liquidargon), and then undergo a cryomilling process at cryogenictemperatures.

Alternatively, in a spray-drying process, a feedstock of thepolyamide(s), the charge control agent, any heat absorber, and/or anyadditional additives may be dissolved or otherwise suspended in acarrier liquid (e.g., solvent). The resulting solution or slurry is thensprayed from an atomizer or spray nozzle along with a heated drying gas(e.g., heated dry air or nitrogen). The heated drying gas can be passedas a co-current or counter-current flow to the spray direction, asdesired.

The sprayed droplets of the solution or slurry rapidly dry to producethe powder form of the part material, preferably with controlledparticle sizes and a narrow particle size distribution. The powder maythen be collected and undergo one or more additional classificationprocesses to attain the desired particle sizes and particle sizedistribution, if required. The flow control agent may also optionally beblended with the powder at this point in the process, if desired.

In a further alternative embodiment, the part material may be formulatedfrom the polyamide(s) with a limited coalescence process, such as theprocess disclosed in Bennett et al., U.S. Pat. No. 5,354,799. Forexample, the constituents of the part material (e.g., the polyamide(s),charge control agent, heat absorber, and/or additional additives) may bedissolved or otherwise suspended in an organic solvent to a suitableconcentration range such as from about 10% to about 20% by weight of thepoylamide(s) in the organic solvent. Examples of suitable organicsolvents include ethyl acetate, propyl acetate, butyl acetate,dichloromethane, methyl ethyl ketone, cyclohexane, toluene, mixturesthereof, and the like.

Separately, a buffered acidic aqueous solution may be preparedcontaining a dispersant such as colloidal silica, and preferably awater-droplet interface promoter, such as poly (adipicacid-co-methylaminoethanol). The organic solvent solution may then beslowly (e.g., incrementally) added to the buffered acidic aqueoussolution while subjecting the whole mixture to high shear mixing, suchas with a homogenizer. This creates droplets of the organic phase ofcontrolled size and size distribution, which are stabilized by thecolloidal silica in the aqueous phase. This mixing preferably continuesuntil droplet growth and creation is completed.

The stabilized solvated droplet suspension may then be passed to a flashevaporator, where the organic solvent may be removed to a condensatetank using applied vacuum. The solid particles of the resulting partmaterial, which remain dispersed in the aqueous phase, may then betransferred to a stirred holding vessel, and the colloidal silica may beremoved, such as with the use of an aqueous sodium hydroxide solution,filtration, and water.

The part material may then be dried to produce its powder form. Ifnecessary, following particle size analysis, the dry powder of the partmaterial may be subjected to further sieving to remove oversizeparticles, and/or classification to remove any level of fines that areconsidered detrimental to subsequent performance in system 10. Thisprocess typically produces the part material in a yield ranging fromabout 90% by weight to about 99% by weight, based on the original amountof the polyamide(s) employed.

Examples of suitable polyamide materials for use in the part materialmay include polyamide homopolymers and copolymers derived from monomersthat include caprolactam, diamines in combination with monomers thatinclude dicarboxylic acids, and mixtures thereof. The diamine monomersand the dicarboxylic acid monomers are each preferably aliphaticmonomers, and more preferably are each acyclic aliphatic monomers.However, in other embodiments, the diamine monomers and/or thedicarboxylic acid monomers may include aromatic or cycloaliphatic groupswhile maintaining crystalline domains. Furthermore, in some embodiments,the semi-crystalline polyamide(s) may include cyclic groups in graftedpendant chains (e.g., maleated groups), as discussed below. Preferredpolyamide homopolymers and copolymers for the semi-crystallinepolyamide(s) may be represented by the following structural formulas:

where R₁, R₂, and R₃ may each be a hydrocarbon chain having 3-12 carbonatoms. The hydrocarbon chains for R₁, R₂, and R₃ may be branched (e.g.,having small alkyl groups, such as methyl groups) or unbranched, andwhich are preferably aliphatic, acyclic, saturated hydrocarbon chains.

As used herein, reference to a repeating unit identifier “n” in apolymer structural formula means that the bracketed formula repeats forn units, where n is a whole number that may vary depending on themolecular weight of the given polymer. Furthermore, the particularstructures of the bracketed formulas may be the same between therepeating units (i.e., a homopolymer) or may be vary between therepeating units (i.e., copolymer). For example, in the above-shownFormula 1, R₁ may be the same structure for each repeating unit toprovide a homopolymer, or may be two or more different structures thatrepeat in an alternating copolymer manner, a random copolymer manner, ablock copolymer manner, a graft copolymer manner (as discussed below),or combinations thereof.

Preferred polyamides include nylon-type materials such aspolycarpolactum (PA6), polyhexamethyleneaidpamide (PA6,6),polyhexamethylenenonamide (PA6,9), polyhexamethylenesebacamide (PA6,10),polyamide 6/12 (PA6,12), polyenantholactum (PA7), polyundecanolactum(PA11), polylaurolactam (PA12), and mixtures thereof. More preferably,the polyamides for the semi-crystalline polyamide(s) include PA6; PA6,6;PA 6,12; PA11; polylaurolactam (PA12), and mixtures thereof.

Similarly, in embodiments in which the semi-crystalline thermoplasticmaterial is a polyethylene material or a polypropylene material, thepart material may be produced with the above-discussed spray-dryingprocess. Alternatively, for polypropylene materials, the part materialmay be formulated with the use of a thermally-induced phase separationtechnique to produce the powder with the controlled particle sizes andnarrow particle size distribution. With either technique, the resultingpowder may also undergo one or more additional classification processesto attain the desired particle sizes and particle size distribution, ifrequired.

Examples of suitable polyethylenes for use in the part material includelow-density polyethylene, medium-density polyethylene, high-densitypolyethylene, and combinations thereof. Furthermore, suitablepolypropylenes for use in the part material include isotacticpolypropylenes, syndiotactic polypropylenes, branched and linearvariations thereof, and combinations thereof.

In embodiments in which the semi-crystalline thermoplastic material is asemi-crystalline polyolefin block copolymer, the part material may beproduced with the above-discussed spray-drying process and/or theabove-discussed limited coalescence process. Examples of suitablesemi-crystalline polyolefin block copolymers for use in the partmaterial include poly(ethylene norbornene), poly(propylene norbornene),poly(ethylene-propylene-norbornene), and combinations thereof.

In embodiments in which the semi-crystalline thermoplastic material is apolyoxymethylene material, the part material may be produced with eitherof the above-discussed milling process or the above-discussedspray-drying process. With either technique, the resulting powder mayalso undergo one or more additional classification processes to attainthe desired particle sizes and particle size distribution, if required.

Examples of suitable polyoxymethylene materials for use in the partmaterial include acetal copolymers, acetal homopolymers, and mixturesthereof, more preferably acetal homopolymers, such as those commerciallyavailable under the tradename “DELRIN” from E.I. du Pont de Nemours andCompany, Wilmington, Del.

Alternatively, in embodiments in which the semi-crystallinethermoplastic material is a polyaryletherketone (PAEK), the partmaterial may be produced with other suitable milling techniques, such asa stirred milling process or a jet milling process. Examples of suitablepolyaryletherketones include polyetherketone (PEK), polyetheretherketone(PEEK), polyetheretherketoneketone (PEEKK),polyetherketoneetherketoneketone (PEKEKK), mixtures thereof, and thelike, and more preferably polyetheretherketone (PEEK), such as thatcommercially available under the tradename “VICTREX” from Victrex plc,Lancashire, UK.

After being formulated, the part material preferably has particle sizesand particle size distributions as discussed above. In some embodiments,the resulting part material may be surface treated with one or moreexternal flow control agents, as discussed above, to increase the powderflow properties of the part material. For example, the part material maybe dry blended under high speed and sheer, preferably at 25° C., withone or more external flow control agents. This uniformly distributes,coats, and partially embeds the flow control agent(s) into theindividual particles of the part material, without significantlyaltering the particle size or particle size distribution.

The formulated part material may then be filled into a cartridge orother suitable container for use with EP engine 12 p in system 10. Forexample, the formulated part material may be supplied in a cartridge,which may be interchangeably connected to a hopper of developmentstation 58. In this embodiment, the formulated part material may befilled into development station 58 for mixing with the carrierparticles, which may be retained in development station 58. Developmentstation 58 may also include standard toner development cartridgecomponents, such as a housing, delivery mechanism, communicationcircuit, and the like.

The carrier particles in development station 58 may be any suitablemagnetized carrier particles for charging the part material, such ascarrier particles having strontium ferrite cores with polymer coatings.The cores are typically larger in size than the particles of the partmaterial, such as averaging from about 20 micrometers to about 40micrometers in diameter. The polymer coatings may vary depending on theQ/M ratios desired for the part material. Examples of suitable polymercoatings include poly(methyl methacrylate) (PMMA) for negative charging,or poly(vinylidene fluoride) (PVDF) for positive charging. Suitableweight ratios of the part material to the carrier particles indevelopment station or cartridge 58 include those discussed above.

Alternatively, development station 58 itself may be an interchangeablecartridge device that retains the supply of the part material. Infurther alternative embodiments, EP engine 12 p itself may be aninterchangeable device that retains the supply of the part material.

When the part material is loaded to system 10, system 10 may thenperform printing operations with the part material to print 3D parts(e.g., 3D part 80), preferably with a suitable support structure (e.g.,support structure 82). The layers of each 3D part are developed from thepart material with EP engine 12 p and transferred to layer transfusionassembly 20, where they are heated and transfused to each other to printthe 3D parts in a layer-by-layer manner using an additive manufacturingtechnique.

In some preferred embodiments, a resulting 3D part is encased laterally(i.e., horizontally to the build plane) in the support structure, asshown in FIG. 4. This is believed to provide good dimensional integrityand surface quality for the 3D part while using a reciprocating buildplaten 68 and a nip roller 70. The resulting 3D part may exhibitvisually observable layers with layer thicknesses depending on thethicknesses of the layers developed by EP engine 12 p and the nippressure at layer transfusion assembly 20. Compositionally, theresulting 3D part includes the part material, such as the copolymer,charge control agent, heat absorber, flow control agent, and/or anyadditional additives.

Property Analysis and Characterization Procedures

Various properties and characteristics of the part and support materialsdescribed herein may be evaluated by various testing procedures asdescribed below:

1. Glass Transition Temperature and Heat Deflection Temperature

The glass transition temperature is determined using the classical ASTMmethod employing Differential Scanning calorimetry (DSC) ASTM D3418-12e1and is reported in degrees Celsius. The test is performed with a DSCanalyzer commercially available under the tradename “SEIKO EXSTAR 6000”from Seiko Instruments, Inc., Tokyo, Japan, with a 10-milligram sampleof the support material copolymer. The data is analyzed using softwarecommercially available under the tradenames “DSC Measurement V 5.7” and“DSC Analysis V5.5”, also from Seiko Instruments, Inc., Tokyo, Japan.The temperature profile for the test includes (i) 25° C. to 160° C.heating rate 10 Kelvin/minute (first heating period), (ii) 160° C. to20° C. cooling rate 10 Kelvin/minute, and (iii) 20° C. to 260° C.heating rate 10 Kelvin/minute (second heating period). The glasstransition temperature is determined using only the heat flowcharacteristics of the second heating period.

The heat deflection temperature is determined pursuant to ASTM D648-07.

2. Particle Size and Particle Size Distribution

Particle sizes and particle size distributions are measured using aparticle size analyzer commercially available under the tradename“COULTER MULTISIZER II ANALYZER” from Beckman Coulter, Inc., Brea,Calif. The particle sizes are measured on a volumetric-basis based onthe D50 particles size, D10 particle size, and D90 particles sizeparameters. For example, a D50 particle size of 10.0 micrometers for asample of particles means that 50% of the particles in the sample arelarger than 10.0 micrometers, and 50% of the particles in the sample aresmaller than 10.0 micrometers. Similarly, a D10 particle size of 9.0micrometers for a sample of particles means that 10% of the particles inthe sample are smaller than 9.0 micrometers. Moreover, a D90 particlesize of 12.0 micrometers for a sample of particles means that 90% of theparticles in the sample are smaller than 12.0 micrometers.

Particle size distributions are determined based on the D90/D50distributions and the D50/D10 distributions. For example, a D50 particlesize of 10.0 micrometers, a D10 particle size of 9.0 micrometers, and aD90 particle size of 12.0 micrometers provides a D90/D50 distribution of1.2, and a D50/D10 distribution of 1.1.

As mentioned above, the geometric standard deviation σ_(g) preferablymeets the criteria pursuant to the above-shown Equation 1, where theD90/D50 distributions and D50/D10 distributions are preferably the samevalue or close to the same value. The “closeness of the D90/D50distributions and D50/D10 distributions are determined by the ratio ofthe distributions. For example, a D90/D50 distribution of 1.2 and aD50/D10 distribution of 1.1 provides a ratio of 1.2/1.1=1.09, or about a9% difference.

3. Triboelectric Charging

The triboelectric or electrostatic charging properties of powder-basedmaterials for use in electrophotography-based additive manufacturingsystems, such as system 10, may be determined with the followingtechnique. A test sample of 7 parts by weight of the powder-basedmaterial is agitated in a clean dry glass bottle with 93 parts by weightof carrier particles. The carrier particles include a magnetized22-micrometer core of strontium ferrite coated with 1.25% by weight of apolymer coating of poly(methyl methacrylate) (PMMA) for negativecharging, or poly(vinylidene fluoride) (PVDF) for positive charging.

The mixture of the powder-based material and the carrier particles isagitated 25° C. on a jar roller for 45 minutes to ensure complete mixingof the carrier particles and the powder-based material, and to ensureequilibration of the Q/M ratios. This mixing simulates the mixingprocess that occurs in a development station of the electrophotographyengine when the part or support materials are added to the carrierparticles.

A sample of the mixture is then quantitatively analyzed with a TEC-3Triboelectric Charge Analyzer (available from Torrey Pines Research,Fairport, N.Y.). This analyzer uses electric fields to strip theelectrostatic powder from the carrier particle surface, and a rotatinghigh-strength, planar multi-pole magnet to constrain the (magnetizableor permanently magnetized) carrier beads to a bottom electrode.

A 0.7-gram sample of the mixture (sample powder and carrier particles)is placed onto a clean stainless steel disc, which serves as the bottomelectrode in a gap plate under an applied field. This bottom plate ismounted and positioned above the rotating multi-pole magnet, and a cleantop plate disc electrode is mounted securely above the bottom plate, andparallel to it, so as to provide a controlled gap of 5 millimetersbetween the top and bottom electrode plates, using insulatingpolytetrafluoroethylene (PTFE under tradename “TEFLON”) spacers at theelectrodes' periphery.

If the powder is expected to charge negatively, a direct-current voltageof +1,500 volts is applied across the electrodes, and the magneticstirrer is activated to rotate at 1500 rpm, so as to gently keep thecarrier and powder under test constrained, but also slightly agitated onthe bottom electrode, during the measurement. Alternatively, if thepowder is expected to charge positively, then a negative bias voltage of−1,500 volts is applied. In either case, the applied electric fieldcauses the powder to strip from the carrier, in the powder/carriermixture, and to transfer to the top electrode, over a defined timeperiod.

The stripped powder under test is deposited on the top electrode, andthe induced accumulated charge on the top plate is measured using anelectrometer. The amount of powder transferred to the top electrode isweighed, and compared to the theoretical percentage in the originalcarrier powder mix. The carrier remains on the bottom plate due to themagnetic forces constraining it.

The total charge on the top plate and the known weight of transferredelectrostatic powder are used to calculate the Q/M ratio of the testpowder, and to also check that all the electrostatic powder hastransferred from the carrier, according to the theoretical amountoriginally mixed with the carrier beads. The time taken for completepowder transfer to the top plate, and the percent efficiency of thepowder transfer process are also measured.

4. Powder Flowability

As discussed above, the part and support materials of the presentdisclosure preferably exhibit good powder flow properties. This reducesor prevents blockage or flow restrictions of the part or supportmaterial during the replenishment feeding, which can otherwise inhibitthe supply of the part or support material to the carrier particles inthe development station. The powder flowability of a sample material isqualitatively measured by visually observing the flowability of thepowder in comparison to commercially-available toners utilized intwo-dimensional electrophotography processes, which are rated as having“good flow” or “very good flow”.

5. Melt Rheology

Preferably, the melt rheologies of the part and support materials aresubstantially the same as the melt rheologies of their respectivecopolymers, and are preferably not detrimentally affected by the otheradditives. Additionally, as discussed above, the part and supportmaterials for use with electrophotography-based additive manufacturingsystems (e.g., system 10) preferably have similar melt rheologies.

Melt rheologies of the part and support materials of the presentdisclosure, and their respective copolymers, are measured based on theirmelt flow indices over a range of temperatures. The melt flow indicesare measured using a rheometer commercially available under thetradename “SHIMADZU CFT-500D” Flowtester Capillary Rheometer fromShimadzu Corporation, Tokyo, Japan. During each test, a 2-gram sample isloaded to the rheometer pursuant to standard operation of the rheometer,and the temperature of the sample is increased to 50° C. to cause aslight compacting of the sample.

The temperature is then increased from 50° C. at a rate of 5° C. perminute, allowing the sample to first soften and then flow. The rheometermeasures the sample viscosity using the flow resistance of the melt toflow through a small die orifice, as a piston of the rheometer is driventhrough a cylinder. The rheometer records the softening point, thetemperature at which flow begins, and the rate at which flow increasesas a result of the temperature increase, until the cylinder is exhaustedof sample melt. The rheometer also calculates the apparent viscosity inPascal-seconds at each temperature point in the ramp. From this data,the apparent viscosity versus temperature profile can be determined.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

1. A part material for printing three-dimensional parts with anelectrophotography-based additive manufacturing system, the partmaterial comprising: a composition comprising: a semi-crystallinethermoplastic material; and a charge control agent; wherein the partmaterial is provided in a powder form having a D50 particle size rangingfrom about 5 micrometers to about 30 micrometers; and wherein the partmaterial is configured for use in the electrophotography-based additivemanufacturing system having a layer transfusion assembly for printingthe three-dimensional parts in a layer-by-layer manner.
 2. The partmaterial of claim 1, wherein the composition further comprises a heatabsorber, wherein the heat absorber constitutes from about 0.5% byweight to about 10% by weight of the part material.
 3. The part materialof claim 1, wherein the D50 particle size ranges from about 10micrometers to about 20 micrometers.
 4. The part material of claim 1,wherein the powder form also has a D901D50 particle size distributionand a D50/D10 particle size distribution each ranging from about 1.00 toabout 1.40.
 5. The part material of claim 1, wherein the charge controlagent is selected from the group consisting of chromium oxy carboxylicacid complexes, zinc oxy carboxylic acid complexes, aluminum oxycarboxylic acid complexes, and mixtures thereof.
 6. The part material ofclaim 1, wherein the charge control agent constitutes from about 0.1% byweight to about 5% by weight of the part material.
 7. The part materialof claim 1, wherein the composition further comprises a flow controlagent constituting from about 0.1% by weight to about 10% by weight ofthe part material.
 8. The part material of claim 1, wherein thesemi-crystalline thermoplastic material comprises a polyamide, apolyethylene, a polypropylene, a polyolefin block copolymer, apolyoxymethylene, a polyaryletherketone, or mixtures thereof.
 9. Thepart material of claim 8, wherein the semi-crystalline thermoplasticmaterial comprises the polyamide, and wherein the polyamide comprisespolycarpolactum (PA6), polyhexamethyleneaidpamide (PA6,6),polyhexamethylenenonamide (PA6,9), polyhexamethylenesebacamide (PA6,10),polyamide 6/12 (PA6,12), polyenantholactum (PA7), polyundecanolactum(PA11), polylaurolactam (PA12), or mixtures thereof.
 10. The partmaterial of claim 9, wherein the polyamide comprises PA6; PA6,6; PA6,12;PA11; polylaurolactam (PA12), or mixtures thereof.
 11. The part materialof claim 8, wherein the semi-crystalline thermoplastic materialcomprises the polyaryletherketone, and wherein the polyaryletherketonecomprises polyetherketone (PEK), polyetheretherketone (PEEK),polyetheretherketoneketone (PEEKK), polyetherketoneetherketoneketone(PEKEKK), or mixtures thereof.
 12. The part material of claim 11,wherein the polyaryletherketone comprises polyetheretherketone (PEEK).13. A method for printing a three-dimensional part with anelectrophotography-based additive manufacturing system having anelectrophotography engine, a transfer medium, and a layer transfusionassembly, the method comprising: providing a part material to theelectrophotography-based additive manufacturing system, the partmaterial compositionally comprising a charge control agent, and asemi-crystalline thermoplastic material, and has a powder form;triboelectrically charging the part material to a Q/M ratio having anegative charge or a positive charge, and a magnitude ranging from about5 micro-Coulombs/gram to about 50 micro-Coulombs/gram; developing layersof the three-dimensional part from the charged part material with theelectrophotography engine; electrostatically attracting the developedlayers from the electrophotography engine to the transfer medium; movingthe attracted layers to the layer transfusion assembly with the transfermedium; and transfusing the moved layers to previously-printed layers ofthe three-dimensional part with the layer transfusion assembly.
 14. Themethod of claim 13, wherein the powder form of the part material has aD50 particle size ranging from about 5 micrometers to about 30micrometers, and a D90/D50 particle size distribution and a D50/D10particle size distribution each ranging from about 1.00 to about 1.40.15. The method of claim 13, wherein the charge control agent constitutesfrom about 0.1% by weight to about 5% by weight of the part material,and wherein the heat absorber constitutes from about 0.5% by weight toabout 10% by weight of the part material.
 16. The method of claim 13,wherein the part material further comprises a flow control agentconstituting from about 0.1% by weight to about 10% by weight of thepart material.
 17. The method of claim 13, wherein the semi-crystallinethermoplastic material comprises a polyamide, a polyethylene, apolypropylene, a polyolefin block copolymer, a polyoxymethylene, apolyaryletherketone, or mixtures thereof.
 18. The method of claim 17,wherein the semi-crystalline thermoplastic material comprises thepolyamide, and wherein the polyamide comprises polycarpolactum (PA6),polyhexamethyleneaidpamide (PA6,6), polyhexamethylenenonamide (PA6,9),polyhexamethylenesebacamide (PA6,10), polyamide 6/12 (PA6,12),polyenantholactum (PA7), polyundecanolactum (PA11), polylaurolactam(PA12), or mixtures thereof.
 19. The method of claim 18, wherein thepolyamide comprises PA6; PA6,6; PA6,12; PA11; polylaurolactam (PA12), ormixtures thereof.
 20. The method of claim 17, wherein thesemi-crystalline thermoplastic material comprises thepolyaryletherketone, and wherein the polyaryletherketone comprisespolyetherketone (PEK), polyetheretherketone (PEEK),polyetheretherketoneketone (PEEKK), polyetherketoneetherketoneketone(PEKEKK), or mixtures thereof.